Guide to UV Measurement
Calibration Techniques
Relative reading UV measurement instruments such as on-line monitors are ‘adjusted' to a certain value (percentage reading, voltage or current output) by the end user. With a new lamp and clean reflector, the user may elect to adjust the display reading to 100% or the output signal to 5 volts. If the desired output signal is 5 volts, you would check the value with a voltmeter and make adjustments to the output signal until the two are equal. The voltmeter at one point or maybe even on a regular basis is ‘calibrated' to a standard to make sure that 5 volts=5 volts.
For absolute reading radiometers the process is a little different. Calibration for these instruments is the process of evaluating and adjusting the instrument to a known standard through either direct comparisons or through an unbroken, documented chain of transfer standards.
NIST (National Institute of Science and Technology, formerly National Bureau of Standards) is the keeper of standards in the United States . Originally founded over 100 years ago as the organization designated to be the keeper of standards for things like weight, volume, time and measurement, NIST has grown and evolved as technology has changed. While NIST still keeps standards, they are also involved in research and providing technical assistance to companies and industries. Other countries have their own national laboratories, and together participate in international comparison programs to ensure equivalency in their standards.
NIST has very accurately determined the “absolute“ output of specific types of lamps and the responses of specific detectors under strict optical and geometric conditions. They sell versions of these lamps and detectors that they certify will give the absolute values, under the same measurement conditions, to within some margin of uncertainty. Radiometer manufactures purchase lamps and detectors from NIST and other sources that calibrated their lamps and detectors with NIST standards. These are known as NIST traceable standards.
Manufacturers then use these standards to calibrate a radiometer that is identical in design to the customer radiometer. This radiometer is referred to as a transfer or working standard, and in turn is used to calibrate customer radiometers under controlled conditions. view a calibration techniques diagram In this manner, a chain of standards and conditions is established, which allows the calibration to be traced to NIST. The readings made by your radiometer are now traceable to the NIST standard and will read very closely - to within a small margin of uncertainty to the NIST sensor under the NIST specified conditions.
When the lamps are not of the same type used in the calibration and the measurement geometries are not those used in the NIST protocol, the readings may differ by more than a few percent. Lamps, especially those used in UV curing, have radically different spectral outputs from the standard lamps. The various types of detectors used in radiometers may have different overall responses to the spectral outputs of these lamps. The diffusers may see things differently and the human factor in placing the sensors at exactly the same distance each time from the source all contribute to difference in measured values from different models of radiometers.
Calibrating an instrument to one type of spectral source (mercury) and then using it under a second source (additive bulb) can lead to small differences in the readings. If you will consistently use the radiometer under a specific lamp source, ask the manufacturer to calibrate the instrument under that type of source.
The critical operational parameter for a radiometer used in process monitoring is not just absolute accuracy but also excellent repeatability. A repeatable radiometer will always give the same values under identical exposure conditions. With confidence in your radiometer's repeatability, when your irradiance reading becomes 20% less than when the lamp was new, you can be assured that there is a 20% output loss, perhaps due to lamp aging, dirt, or reflector problems.